This invention relates to photovoltaic devices, commonly known as solar cells, capable of converting solar radiation into useable electrical energy. More specifically, this invention relates to amorphous silicon solar cells.
The photovoltage, photocurrent, and lifetime of solar cells and especially amorphous silicon solar cells tends to decrease with increasing cell temperature. Solar radiation having an energy less than the bandgap energy of the semiconductor material which is absorbed by the solar cell tends to raise the temperature of the solar cell and lowers the electrical output and lifetime of the cell. Typically, infrared radiation, i.e., light having a wavelength longer than about 0.80 micrometers (.mu.m) has an energy which is less than the bandgap energy of amorphous silicon. Absorption of the infrared radiation by the solar cell degrades the performance and shortens the useful life of the solar cell.
U.S. Pat. No. 3,888,698 to Lindmayer et al describes a solar cell with a transparent back electrode which permits radiation of less than the bandgap energy, i.e., infrared radiation, to pass through the cell without absorption and heating of the device. Lindmayer et al's solution is acceptable for single crystalline silicon or other semiconductor materials where the active region of the solar cell can be made thick enough to absorb all of the light with energies greater than the bandgap energy of the semiconductor material. However, with amorphous silicon, the minority carrier lifetime is significantly less than in single crystalline silicon and the devices are sufficiently thin so that a portion of the light having an energy greater than the bandgap energy of the amorphous silicon can pass out of the solar cell along with the infrared radiation. Thus, it would be highly desirable to have a device which is transmissive to infrared radiation but reflects back into the active region of the cell solar radiation having an energy which is greater than the bandgap energy of the active region of amorphous silicon.